1. Field of the Invention
The present invention relates to a method of optically coupling an optical fiber to a waveguide formed on a substrate, and an optical device which includes the substrate whose waveguide is coupled to the optical fiber.
2. Discussion of the Prior Art
In recent years, interest in the field of opto-electronics has been much enhanced, by the development of various optical integrated circuits and solid optical deflectors, which include optical elements such as light-emitting elements, light converging elements, optical deflector elements, optical amplifier elements or light-sensitive elements. Generally, such optical elements are integrated on a suitable substrate which is formed with an optical waveguide for guiding a light beam therethrough.
On the other hand, optical information or signal in the form of a light beam is transmitted to or from the waveguide of the substrate, through an optical fiber or fibers. To this end, the optical fiber is optically coupled to the waveguide. However, methods presently available for optically coupling the optical fiber to the waveguide of the substrate suffer from various problems. Some known methods will be described by reference to FIGS. 20-22.
A first known method is shown in FIG. 20, wherein a suitably sized rectangular groove 112 is formed in one of opposite major surfaces of an optical IC substrate 110, such that the groove 112 is open at an end face 110a of the substrate 110. An end portion of an optical fiber 114 is positioned in the groove 112 and secured therein with a suitable bonding agent such as an ultraviolet-curable resin. Thus, the optical fiber 114 is connected to the substrate 110. This method is advantageous in that the end of a core 114a of the optical fiber 114 may be accurately positioned with respect to an exposed portion 118a of a waveguide 118 which is exposed to the inner or closed end portion of the groove 112. However, since the substrate 110 is formed of a relatively brittle material such as a glass and lithium niobate, which is hard to machine, it is generally difficult to form the groove 112 by machining. In other words, the substrate tends to easily crack or chip during a machining operation to form the groove 112, and consequently the ratio of reject of the substrate is comparatively high.
There is shown in FIG. 21 a second known method in which a light beam transmitted through the optical fiber 114 is converged by a converging lens 116, so that the converged light beam is incident upon the ground end face 118a of the waveguide 118 on the substrate 110. This method using the converging lens 116 leads to a relatively large-sized optical device, a relatively high cost of manufacture of the device. Further, the optical device suffers from difficulty in establishing alignment of the optical axis.
A third known method is illustrated in FIG. 22, wherein two rutile prisms 120, 120 consisting of titanium oxide (TiO.sub.2) are formed on the substrate 110 such that the rutile prisms 120 are optically coupled to the waveguide 118. An input light beam incident upon the input side prism 120 is directed to a predetermined point along the length of the waveguide 118. The light beam travelling through the waveguide 118 is incident upon the output side prism 120, from which the beam is emitted as an output light beam from the substrate. The present method is not economically justified because of a relatively high cost of the rutile prisms 120. Further, the prisms 120 protrude from the surface of the substrate 110, increasing a required space for the optical device, and complicating the optical system involved.
Also known is a method wherein a diffraction grating such as a Fresnel lens is formed in the waveguide of the substrate, so that the grating lens serves to convert an incident plane wave into a cylindrical wave by means of diffraction. However, this method is difficult to practice, requiring complicated process steps and resulting in a considerably high cost of manufacture of the optical device. Moreover, the optical device produced has a high degree of dependence upon the wavelength of a light beam, and the alignment of the optical axis is difficult.
There exists a recognized problem that the cracking or chipping of the substrate during a machining operation to form the groove 112, and light dispersion at an optical interface between the end of the optical fiber and the end face of the groove 112 cause a significant loss of optical energy. In view of this problem, a method of forming a groove in the substrate is proposed according to Japanese Patent Application No. 61-288331 filed in the name of the assignee of the present application. In this method, a cutting tool having a pyramidal cutting tip is moved along the groove to be formed in the substrate, while the tool is rotated about its axis. This cutting procedure permits the groove to be readily cut over a desired length, without the conventionally encountered chipping of the substrate, even where the substrate is made of a brittle material such as a glass or lithium niobate. Since the cutting tip has a pyramidal shape, however, the formed groove has an inclined end face at the closed end, at which the cutting motion of the tool is terminated. Therefore, if the end face of the optical fiber to be coupled to the substrate is perpendicular to the length of the fiber, there is left a gap indicated at S in FIG. 9, between the flat end face of the fiber and an exposed face of the waveguide which is exposed to the closed end of the groove. This gap S causes dispersion of the light emitted from the fiber, whereby the optical transmission efficiency is accordingly reduced.